Conventionally, there have been motor control devices for controlling the operation of a motor (brushless motor) through the supply of a drive power of three phases (U, V, W). These motor control devices include, for example, a device for supplying drive power by switching the energized phase and the direction of the energization for every 60 degrees, that is, the energized pattern in terms of the electrical angle of the motor which is the object of the control as shown in FIG. 4 (see, for example, Patent Document 1).
In the case where there are two energized phases (drive phases) in the driving of the motor through such an energization with rectangular waves, the voltage on the drive phase power supply side becomes a high voltage HV, the voltage on the drive phase grounded side becomes a low voltage LV, and the voltage of the non-drive phase becomes a middle voltage MV. That is, the following relational expression is theoretically established between the respective phase voltages. The respective phase voltages are monitored to determine whether this relational expression is established, so that a break in a power supply line to the motor can be detected.HV+LV−2×MV=0  (1)    Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-295688